Multi-channel optical transmission equipment deployed in communications networks must be monitored in a manner which allows system performance and the location of faults to be ascertained quickly and reliably. Typically, this is accomplished in regenerative-type optical transmission systems by electrically monitoring each channel at each regeneration site. However, regenerative-type optical transmission systems are expected to be replaced by systems which utilize chains of optical amplifiers arranged in stages to periodically boost an optical signal as it travels over the network. Electrical monitoring is impractical in these systems because the optical signal is not generally converted to an electrical signal until it reaches the receiver.
Ideally, a monitoring technique for transmission systems using chains of optical amplifiers should allow a determination of the performance of each of the individual channels amplified by each optical amplifier in the chain, at each stage of amplification. The ability to determine and indication of the optical signal-to-noise ratio ("CO-SNR")--a key performance parameter--would be a particularly desirable monitoring feature. It would also be desirable to be able to isolate faults to a specific channel of a particular optical amplifier in the chain.
Some monitoring and fault location techniques require that channel performance be measured at each amplification stage using complex and expensive measuring equipment. Other techniques may require the performance of complex calibrations of various electrical and optical components in the network.